I have noticed the same thing. Looking at the Windows DNS cache, it's evident that (when the internet dns server sends both A and AAAA records) dnsmasq returns AAAA records regardless of not choosing ipv6 in pi hole setup.
Look for "::" in the windows cache
ipconfig /displaydns
or ipconfig /displaydns > cache.txt
I have never used ipv6, pihole has worked perfectly since I installed it last year and all I've done is kept it up to date. I don't have any ipv6 options enabled and they are all greyed out.